docs: document the dashboard's templates workflow and add a Dashboard section to the user guide

[AaronProbha18]

Changes

• Added a Live Dashboard entry to Core Features on the homepage (src/index.md), covering browsing/filtering/renaming sessions and running pulled templates from the browser.
• Added a Dashboard section to the Quickstart (src/content/quickstart.md), with the fenn dashboard start command and a brief description of what it does.
• Added a new Dashboard group under User Guide in mkdocs.yml, with three new pages under content/user_guide/dashboard/:
  • index.md — starting the dashboard (--port, --log-dir, --debug), the pyfenn.com sign-in flow, and a page overview
  • sessions.md — browsing, filtering (project/status/date range/sort), renaming, archiving vs. deleting
  • templates.md — how the local template registry is populated by fenn pull, the FENN_TEMPLATES_REGISTRY_PATH env var, and the Run → poll → redirect flow

Addressed issue: pyfenn/fenn#247

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